The "Localhost" Trap: Networking Microservices in Docker Compose

The "It Works On My Machine" Illusion You've built a beautiful, decoupled microservices architecture. Your Node/Express API runs smoothly on port 3000, and your React frontend fetches data perfectly from http://localhost:3000/api . You containerize both services, spin them up with Docker Compose, and suddenly the frontend crashes with a Connection Refused error. What happened? You fell into the localhost trap. Understanding Docker's Internal Network When running services locally without Docker,